1. Field of the Invention
The present invention relates to a method for integrating epitaxial semiconductor device layers with arbitrary host substrates, wherein the epilayers are grown on a growth substrate in an inverse order and joined to a host substrate in a manner that alleviates removal of the epilayers from the growth substrate prior to bonding the epilayers to the host substrate, and bonds the epilayers to the host substrate in a manner which maximizes the strength, yield, and the thermal conductivity of the resulting semiconductor device.
2. Description of the Prior Art
Methods for the integration of epitaxial quality semiconductor device layers with arbitrary host substrates are generally known in the art. These methods of integration are beneficial since device epilayers can be combined with arbitrary substrates which have better optical, mechanical or thermal properties. One type of integration may be accomplished by a method known as lattice-mismatched heteroepitaxial growth. For example, GaAs device epilayers are grown on Si substrates. This method, however, has a significant disadvantage, in that the crystal quality of the resulting material is often insufficient for some microelectronic applications. An alternative method which seeks to preserve the high material quality of lattice-matched device epilayers, while still allowing the integration of device epilayers with other substrates, is the epitaxial liftoff process (ELO). Examples of the ELO process are disclosed in the following and numerous other publications: "Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates", Appl. Phys. Lett. 56 pp. 2419-2421, 1990; and "Alignable Epitaxial Liftoff of GaAs Materials with Selective Deposition Using Polyimide Diaphragms", IEEE Trans. Phot. Techn. Lett. 3 p. 1123, 1991.
In epitaxial liftoff, a thin sacrificial (AlAs) or etch-stop layer 22 is grown on a growth substrate (GaAs) 20, as illustrated in FIG. 1a. Device epilayers are grown sequentially on the sacrificial layer beginning with a first device epilayer 24 and followed by a second device epilayer 26 until all epilayers are grown as required by the type of semiconductor device, shown in FIG. 1b. Next, the device epilayers are covered with a wax-like material (Apiezon W) 28 prior to "liftoff", as shown in FIG. 1c. The GaAs epilayers are then "lifted off" from the growth substrate 20 by etching away the sacrificial layer 22, as illustrated in FIG. 1d. Following the epilayer "lift off", the GaAs epilayers are maintained suspended in the wax-like material 28 until they are bonded to the new host substrate 30, as shown in FIGS. 1e and 1f. Finally, the wax-like material 28 is removed, as shown in FIG. 1g. Although epitaxial liftoff provides, most significantly, a means to integrate high quality epilayers with "best match" host substrates, the wax-like material used for the transfer of epilayers from growth to host substrates presents some problems. The wax-like material does not provide sufficient mechanical protection of the device epilayers, which in turn can cause electronic yield loss. The wax-like material needs to be dissolved in trichloroethylene and the use of trichloroethylene has associated environmental concerns. The use of the wax-like material also makes it impossible to align the "lifted off" epilayers to any pattern on the host substrate because of the opaque nature of the wax-like material.
In epitaxial liftoff processes, "lifted off" epilayers must be bonded to the new host substrate. Various methods are known for bonding device epilayers to host substrates, for example, as disclosed in "Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates", Appl. Phys. Lett. 56 pp.2419-2421, 1990. This publication describes the "Van der Waals" bonding process where epilayers, while suspended in wax-like material, are rinsed with de-ionized water and partially dried. A very small amount of water is left behind. The surface tension of the de-ionized water acts initially to pull the epilayers down onto the host substrate. Then pressure must be applied to remove all but one optical fringe thickness of water. The remaining trace of water evaporates, leaving a permanent bond between epilayers and substrate. While the "Van der Waals" bonding process can be sufficient to achieve closer coupling between the epilayers and host substrate, and allow for further thermal processing or better thermal conduction by eliminating the explicit need for bonding adhesives, "Van der Waals" bonds tend to be much weaker and less reliable than other bonds.
The publication, "Dielectrically-Bonded Long Wavelength Vertical Cavity Laser on GaAs Substrates Using Strain-Compensated Multiple Quantum Wells", IEEE Phot. Techn. Lett. 6 p. 1400, 1994 discloses an alternate bonding technique. This approach relies on an intermediate dielectric (glass) layer to bond compound semiconductor devices with other substrates. The bonding solution is a glass-forming aqueous mixture. This mixture is spun on both the semiconductor device and the host substrate. Immediately after spinning, the device and host substrate are placed face to face where they adhere to each other. The resulting pair is then baked and the spin-on solution turns into a strong layer of glass sandwiched between the device and the host substrate. This method affords a strong bond with low optical loss but may not be optimal when bonding small device epilayers with substrates. Spinning a solution uniformly over a large and symmetric surface is easier than spinning the same solution on a small surface. In addition, dielectric bonds have very low thermal conductivity and are therefore not useful to make contact with thermally highly conductive host substrates.
Unfortunately, the "lift off" method disclosed above requires the use of a wax-like material to remove the device epilayers from the growth substrate and to transfer the device epilayers to the new host substrate. The use of such material poses a number of problems. First, additional processing steps are required for both the application and removal of the wax-like material. Regarding the removal of the wax-like material, the solution required to dissolve the wax-like material poses environmental concerns. Additionally, the fragile device epilayers, while suspended in the wax-like material, are not provided with sufficient protection from mechanical damage, thus potentially jeopardizing the electronic yield of the semiconductor device. Finally, as mentioned above, the wax-like material makes alignment of the device epilayers with patterns on host substrates difficult because of the opaqueness of the wax-like material.
There are also problems associated with the disclosed bonding methods. The "Van der Waals" bonding method, as described above, does not result in strong device epilayer to host substrate bonds, therefore compromising the overall strength of the resulting semiconductor device. Dielectric bonding methods require more complicated "spin on" bonding techniques and the spinning process does not produce the best results on small surface areas. Adhesive bonding methods typically produce strong bonds, but the adhesive material may contribute to low thermal conductance.
For the aforementioned reasons, there is a need for a "liftoff" method which does not require the wax-like material. There is also a need for a bonding method which produces robust semiconductor devices with optimal thermal contact to the host substrate of the semiconductor devices.